1. Field of the Invention
The invention concerns a variable geometry antenna reflector adapted to provide from a spacecraft such as a satellite a transmit and/or receive coverage zone on the ground having a non-circular contour, for example a contour surrounding a country or a group of countries (see FIG. 1), that is required to be modifiable during the service life of the spacecraft. In practice this means an in-orbit reconfigurable shaped contour beam antenna reflector or, for short, an in-service reconfigurable antenna reflector.
Although the invention is primarily directed to a spacecraft application, it is to be understood that it is of more general application to any antenna reflector where it is necessary to be able to change the shaped of the beam in service without changing the reflector (large high-precision telescopes, for example).
2. Description of the Prior Art
The conventional way to obtain a shaped contour beam is to use multiple feeds illuminating a single or double offset reflector system according to an appropriate law. The beam is obtained by exciting the feed elements with optimized phase and amplitude by means of a signal forming network composed of waveguides ("beam forming network").
Another way to obtain a radiation pattern having the required contour is to use a single feed associated with a shaped surface reflector system (by which is meant a shape having a specific geometry, for example a non-quadratic geometry like that of FIG. 2). Variations in the optical pat between the feed and the various points on the reflector make it possible to generate a diagram whose phase and amplitude match the characteristics of the required radiation diagram.
Because the service life of satellites is being increased, it is becoming necessary to be able to modify the beam shape in orbit in order to compensate for variations in orbital position and to meet new service constraints. Reconfigurable antenna systems are conventionally obtained by integrating into the beam forming network power splitters and phase-shifters with variable characteristics. This renders the multiple feed highly complex which introduces radio frequency power losses, the risk of passive intermodulation products in the case of a transmit antenna, constraining thermal regulation requirements for the satellite platform and a mass penalty.
An alternative solution to the problem of reconfiguring a reflector antenna in orbit is to employ a system of one or more reflectors whose reflective surfaces are deformable so that the radiation diagram can be modified.
The feasibility of this approach has already been investigated by CLARRICOATS et al. See in particular "A reconfigurable mesh reflector antenna" by P. J. B. CLARRICOATS, Z. HAI, R. C. BROWN, G. T. POULTON & G. CRONE published in ICAP Conference, April 1989, or "The design and testing of reconfigurable reflector antennas" by P. J. B. CLARRICOATS, R. C. BROWN, G. E. CRONE, Z. HAI, G. T. POULTON & P. J. WILSON published in ESA Workshop for antenna technology, November 1989. However, the proposed concept uses a gold-plated molybdenum knitted mesh reflective surface shaped point by point using an array of strings tensioned by a system of pulleys controlled by stepper motors.
From the mechanical and geometrical points of view the deformable surface behaves like a membrane with the result that the reflective surface has numerous singularities (see FIG. 3, for example). Consequently, obtaining the precise profile required of the reflector despite such singularities calls for a large number of control points.
An object of the invention is to alleviate the aforementioned disadvantages by minimizing the presence of artifacts such as the aforementioned singularities at the surface of an in-service reconfigurable antenna.
The solution put forward for obtaining a regular surface resides in the use of a reflective and elastically deformable skin which is stiff in bending but sufficiently flexible at its interfaces with the supporting structure or the actuators to limit the deformation forces and energy.